The iontophoresis through a intracellular micropipette of radioactive chemicals followed by autoradiography makes possible the adequate test of a number of hypotheses relevant to cytoplasmic and synaptic mechanisms within single neurons. Tested in the previous grant period was the effect of antidromic stimulation on uptake of 3H glycine and on the rate of axoplasmic flow. Stimulation increases uptake, but does not alter the rate of axoplasmic flow. The rate of distribution throughout the soma-dendritic complex is close to the rate of axoplasmic flow. The mechanism of this soma-dendritic distribution is under study. The morphology of the motoneuron as shown by autoradiography is identical to that found in Golgi preparations. After intracellular iontophoresis of 3H glycine in motoneurons a few adjacent neurons are labelled, but to a lesser degree. With only one 3H glycine injected neuron, one might locate three lightly labelled neurons within 1000 mu. So far it has not been possible to determine the mechanism by which they are labelled. In the previous three years, methods have been developed to trace the axonal collateral thought to be responsible for this surprising finding. Technical problems included fixation techniques to reduce somal shrinkage in order to trace the terminal portions of the axon collaterals, cutting thin enough sections to allow the Beta- radiation of 3H (penetration 2.8 mu within the fine axon collaterals (diameters equals approximately 0.5 mu) to show on the autoradiography, and collection of every serial section to provide for accurate reconstruction of axon collateral arborizations. If the lightly labelled neuron is labelled through synaptic sites from the intracellular injected cell, then the degree of label is a function of the synaptic drive: the number of synapses and the frequency of synaptic transmission. Experimental material is now undergoing histological preparation. Further objectives will be to study the distribution of label within the soma, dendrite, axon, and axon collaterals by means of the electron microscope, to apply the method to pyramidal cells within the cerebral cortex, and finally to investigate other amino acids thought to be of functional significance in the central nervous system.